Influence Of Oxygen And Trace Water On Power Frequency Insulation And Decomposition Characteristics Of C₄F₇N/CO₂ Gas Mixtures

C₄F₇N/CO₂ gas mixtures is a potential alternative gas for environmental protection,but its chemical stability is weaker than SF₆,and the continuous decomposition phenomenon under the action of latent defects such as partial discharge cannot be ignored.At present,there are few studies on the decomposition regulation of C₄F₇N/CO₂ gas mixtures,which cannot provide a reliable theoretical basis for the improvement of its chemical stability.In this paper,based on the actual operating conditions of gas insulation equipment,the correlation between the decomposition product content and the contents of oxygen and trace water under common latent partial discharge defects is established,and the influence mechanism of oxygen and trace water on the decomposition process is revealed,a method for suppressing the decomposition of C₄F₇N/CO₂ gas mixtures based on oxygen addition and trace water controlling.The influence of chemical stability factors oxygen and trace water on the insulation performance of C₄F₇N/CO₂ gas mixtures was obtained by carrying out power frequency breakdown test synchronously.The research results provide theoretical support and technical basis for the reliability improvement of long-term engineering application of C₄F₇N/CO₂ mixed gas insulation equipment.The main achievements of this paper are as follows:（1）The power frequency breakdown tests of C₄F₇N/CO₂ gas mixtures under quasi-uniform,slightly non-uniform and extremely non-uniform electric fields were carried out by using spherical plate,rod plate and needle plate electrode models,respectively.The power frequency breakdown voltages of C₄F₇N/CO₂ gas mixtures with five kinds of oxygen and five kinds of trace water contents under five kinds of electric fields and six kinds of air pressures were obtained.The correlation between the power frequency breakdown voltage and air pressure,oxygen and trace water contents under different uniformity electric fields was established,and it was found that oxygen and water water had a certain negative impact on the power frequency breakdown voltage of the C₄F₇N/CO₂ gas mixtures.Moreover,the negative effects under the quasi-uniform and slightly non-uniform electric fields commonly found in the equipment are more obvious,the oxygen addition in the gas mixtures should be controlled and the trace water content should be closely concerned.（2）The influence rule and mechanism of oxygen and trace water on the decomposition characteristics of C₄F₇N/CO₂ gas mixtures under suspension and corona discharge conditions were studied,and the relationship between the generation rate of main decomposition products and the content of oxygen and trace water was obtained.Based on the density functional theory,the decomposition models of C₄F₇N/CO₂/O₂ and C₄F₇N/CO₂/trace water gas mixtures were established and simulated,and the generation path and energy characteristics of the main decomposition products were obtained.It is clarified that oxygen,O and OH radicals are the main factors affecting the generation rate of main decomposition products,it was found that the reaction between O₂ and free radicals formed a competitive relationship with the reaction of free radicals to produce decomposition products,which inhibits the formation of the main decomposition products,O radical was generated by oxygen decomposition and the reaction between oxygen and free radicals,the reaction between O radicals and C₄F₇N promotes the formation of free radicals and the reaction of free radicals to produce decomposition products,trace water plays a catalytic role in the formation of main decomposition products by its own decomposition and reaction with free radicals to generate OH radicals.（3）The comprehensive influence of oxygen and trace water on the insulation performance and decomposition characteristics of C₄F₇N/CO₂ gas mixtures under power frequency voltage was analyzed,based on the Wagner equation theory,the liquefaction temperature of C₄F₇N gas mixtures under different pressures was calculated,and the engineering application reliability improvement method of C₄F₇N/CO₂ gas mixtures under typical latent partial discharge defects was obtained,the optimization application method of C₄F₇N/CO₂/O₂ ternary mixed gas and the trace water control index of C₄F₇N/CO₂ binary mixed gas were proposed.